Laser Doppler Imaging (LDI) is a non-contact imaging modality based on the coherence properties of light. This imaging modality mainly developed thanks to new detector technology, software and the availability of appropriate laser sources. The performance improved steadily over the last two decades from the initial proposals based on a scanning instrument towards a state of the art instrument for medicine mainly due to a parallel imaging instrument based on CMOS array detectors.
LDI is a coherent imaging technique that allows imaging of moving particles especially cells in blood flow with a good discrimination between perfusion, flow velocities and the concentration of moving particles i.e. mainly the key flow parameters of erythrocytes.
In conventional scanning LDI the back reflected light from a biological sample or the skin or the organ is detected with a single point detector. This light contains the coherent superposition of a back reflected component from non-moving parts and a back reflected light component from moving particles which causes detectable light fluctuations and allows the extraction of maps of flow velocities, concentration of flow particles or the so-called perfusion as the product of flow velocity times flow particle concentration.
In parallel LDI, the signal results from the interference or coherent superposition between a coherent back-scattered light field originating from the coherently illuminated sample of non-moving parts and the coherent back-scattered light field from moving particles contained in the illuminated volume. A 2D array of random-pixel-access integrating photo detectors (e.g. integrating CMOS image sensor) is used to measure the intensity variations at each individual pixel. The average amplitude and the mean frequency of the measured signal contain information about concentration and speed of moving blood cells. Finally maps of flow velocities, concentration of flow particles or the so-called perfusion as the product of flow velocity times flow particle concentration can be displayed as an image.
Anomalous changes in peripheral blood flow are known to be an indicator of various health disorders in the human organism. Laser Doppler Perfusion Imaging (LDPI) is an imaging technique successfully used for visualization of two-dimensional (2D) micro-vascular flow-maps in a number of clinical settings including investigations of e.g. peripheral vascular diseases, skin irritants, diabetes, burns and organ transplants. This method is non-invasive because it involves no physical contact; the risk of infection and additional discomfort is completely avoided.
The technical principle is based on the Doppler effect wherein the light scattered by moving particles, e.g. blood cells, leads to a slight frequency shift, which can be measured by a heterodyne detector. A 2D flow map is obtained by means of sequential measurements from a plurality of predetermined points. In classical LDPI systems this is achieved by scanning the area of interest with a narrow collimated or focused laser beam. However this scanning approach is time-consuming and suffers from artifacts caused by the mechanical steering of the probing laser beam. In current commercial available LDPI systems these artifacts are circumvented on an expense of imaging time.
For those skilled in the art an alternative full-field flow imaging techniques using speckle contrast analysis is also known. For real-time full-field imaging, the exposure time is used as a parameter to measure relative perfusion changes by means of laser speckle imaging technique. The advantage of this approach is a fast image acquisition, which is achieved at an expense of spatial resolution. However, the technique can be hardly exploited for flow measurements, where either concentration or speed of moving particles is not known in advance. Both said parameters influence the system response in the same manner, and, generally, the cause of the contrast decay is not obvious. Also the system response is not linear to the velocity since a finite camera integration time influences the measurement.
In order to decrease the imaging time for parallel LDI, a parallel detection scheme has been employed increasing the imaging speed by a factor proportional to the number of channels working in parallel. A 2D matrix of photo-detectors is a suitable detection device for that purpose.
Recently Serov et al. [A. Serov, W. Steenbergen, F. F. M. de Mul, “Laser Doppler perfusion imaging with a complimentary metal oxide semiconductor image sensor”, Opt. Left. 25, 300-302 (2002)] suggested a new approach on parallel laser Doppler imaging: a non-integrating true-random-addressing CMOS image sensor was used to detect Doppler signal from a plurality of points on the sample illuminated with a divergent laser beam. Here the mechanical scanning is substituted by the photoelectrical scan resulting in a faster imaging speed.
The use of non-integrating 2D array of photo-detectors for the purpose of laser Doppler has been disclosed in three publications.
A first publication U.S. Pat. No. 6,263,227: “Apparatus for imaging microvascular blood flow”. The concept of using a 1D or 2D matrix of conventional photo detectors is described. The imager can work in two modes - scanning or static. In the scanning mode a laser line is projected on the area of interest. The signals from the illuminated areas are detected by 1D matrix of photo detectors. By scanning the illumination laser light over the area of interest, a 2D perfusion map is obtained. In the static mode the whole area of interest is illuminated by an expanded laser beam or by light exiting an optical fiber. The Doppler signal is measured by 2D matrix of photo detectors. Each photo detector has its own electronics for signal processing. A CCD camera is used to observe the object of interest. The perfusion maps are superimposed on the photographic image obtained with the CCD.
A second publication WO03063677: “Laser Doppler perfusion imaging with a plurality of beams” and a third publication GB2413022:” Laser Doppler perfusion imaging using a two-dimensional random access high pixel readout rate image sensor”. Here, a structured illumination is used for illuminating a plurality of points or an area of interest. The Doppler signal from the illuminated areas is detected with 2D matrix of non-integrating (direct-access) photo detectors. For the detection, the use of random-access-fast-pixels-readout CMOS image sensor is claimed. A single CMOS image sensor is used for detecting the Doppler signal and to obtain a photographic image of the object of interest.
All previously mentioned publications describe arrays of non-integrating detectors that measure instantaneous changes of the photocurrent through the detector. Besides the fact that both publications disclose imaging systems based on integrating detectors, both documents use a true laser Doppler technique to measure the flow.
Laser speckle imaging (LSI) is an alternative technique to access blood flow in tissue. This technique has never been patented but was described in scientific publications; for a review see [J. D. Briers, “Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging”, Physiol. Meas. 22, R35-R66 (2001)]. This technique is based on the image speckle contrast analysis. Various modifications of this technique were reported but those modifications are mainly focused on the signal processing part rather the measurement principal, which is virtually the same for all variants. The LSI system obtains flow-related information by measuring the contrast of the image speckles formed by the detected laser light. If the sample consists or contains moving particles, e.g. blood cells, the speckle pattern fluctuates. The measured contrast is related to the flow parameters (such as speed and concentration of moving particles) of the investigated object. The contrast value is estimated for a certain integration time (exposure time) of the sensor. The faster the speckle pattern fluctuations the lower the contrast value measured at a given exposure time. The control unit defines the exposure time of the image sensor to determine the range of the measured flow-related data related to the image contrast in Laser Speckle Imaging mode. Here, the integration time defines the range of measured speeds. The use of integrating image detectors is mandatory. Until now only the use of CCD type image sensors were reported for the technique.
The LSI is true real-time imaging technique, however as explained above the LSI signal approach cannot discriminate between concentration of flowing particles and their speed. The laser Doppler imaging provides more information as that the LSI method since with laser Doppler the concentration and speed signals can be measured independently. In LSI those signals are always intricately mixed i.e. it is impossible to deduce from speckle contrast changes in concentration or in speed of moving particles Generally, the LSI approach alone is more likely to be a qualitative indicator of blood flow but not a measuring instrument to accurately investigate physiological phenomena. However as claimed in this invention both concepts have the potential to be used in combination, which may lead to an even better overall performance for a perfusion imaging.
Summarizing the above considerations, we conclude:                a) LDI discriminates between the flow parameters (speed and concentration of moving particles). However LDI is perceived as a slower imaging modality in comparison to LSI.        b) LSI is a fast imaging technique, however the results obtained with this technique have not the information content as LDI i.e. does not allow acquiring particles speed and concentration independently.        